Start with connected load
List every load the generator must support, including motors, lighting, HVAC equipment, pumps, controls, and IT equipment. Convert each load to kW or kVA using the available nameplate data.
For three-phase equipment, calculate apparent power from voltage and current when kVA is not listed. Then apply realistic demand factors so the generator is sized for actual operation, not an impossible all-on condition.
Account for starting current
Motor starting can demand several times normal running current. If several motors start together, the generator may need extra capacity to prevent excessive voltage dip.
Soft starters, VFDs, sequenced controls, and reduced-voltage starting can reduce the required generator size by lowering inrush current.
- Identify the largest motor load
- Check locked-rotor or starting kVA where available
- Sequence starts when practical
- Add margin for future load growth
Choose a practical margin
A generator should not be selected right at the calculated load unless the load profile is very well understood. Reserve capacity helps handle transients, heat, altitude derating, and future additions.
For many facilities, a 15 to 25 percent margin is a sensible starting point, but final selection should follow manufacturer data and local electrical requirements.